jarrod0987

The voltage between the two driver bases won't change much when you replace R42.But the current through the drivers will

Understood.

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Look upon C12 (and all the bias the parts attached to it) as an elevator.It can rise up and down, almost to the 60th floor and 60 floors down.But the height of the elevator always stays the same.

OK, But what voltage should I see across C12? How do i know when I have it set right?

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A fusable resistor is just a normal resistor that burns up in a controlled way when something is wrong.Up to you to use fusables or not.

Understood. Just wanted to know if fusible was important here or just a cost saver.

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R9 should be 1k. I assume you measured that disconnected, not in circuit.A wrong value here could upset the balance of the amp.

I measured it just in circuit for an initial reading. going to pull it and test it right tomorrow.

Somewhere I think I was advised the voltages at the bases of the transistors listed in the drawing needed to be exact and so shouldn't the R29,30, and 34 voltages drops etc. Is that right? They are all high.

OK, But what voltage should I see across C12? How do i know when I have it set right?

The voltage across C12 should be ~~~2volt.That voltage is there to put the power stage (Q17,20,21,22) on the edge of conducting.That threshold point is temperature dependent (power/music).So the bias circuit measures the temp, and adjust that voltage.That variation in voltage is not huge, but even the slightest change makes a big difference to the power stage current.How do you know it's right? Bias current causes a volt drop across the four 0.1ohm resistors.Set the bias to 20-40mA when the amp is fixed. Explained in previous posts.

Somewhere I think I was advised the voltages at the bases of the transistors listed in the drawing needed to be exact and so shouldn't the R29,30, and 34 voltages drops etc. Is that right? They are all high.

The whole voltage stage is a balancing act.Start measuring voltage across R12/R13 (~1.55volt).Q4 converts that into a current (0.5mA).That is converted into a voltage across R34 > R30 > R29.! the -1.55v listed next to R34 is the voltage across D13 and R34 !Leo..

jarrod0987

I have 2.151v. Will that be acceptable? The numbers on the drawing would give 2.2v.

Also...My understanding of bias is that it is DC and you measure it as DC with 1khz test signal or anything.

I am a little confused because I thought this was an AB class amplifier and it's whole benefit was that there was no current through the emitters of the final stage when there was no signal? I Guess .5ma is still basically nothing. I will try to make this adjustment tonight as it seems like there isn't much wrong now. I do have to check that 1K resistor out though.

Thank You

UPDATE: So I checked that 1K resistor. It's fine out of circuit. I rebuilt the amp.

I was to scared to try to turn it on without the heat sink on because I have blown it so much. I can't get to the temporary 220 Ohm resistors when the heat sink is installed. I really am unclear weather those various voltages stated on the drawing must be exact or not. At any rate I have checked every single component on the board now and replaced a few bad things since the last time I tried to adjust anything. In fact...I could get any adjustments out of R21 because I did not know Q3 was bad last time around. I also replaced that burned out fuse which was probably causing the problem with the outputs blowing. I know I'm not supposed to put the outputs back in until I'm sure it's right. Normally I would put a signal in and look at the resistors I installed but I'm to chicken to do it with no heat sink and I can't get to them with one. I felt pretty confident it should be proper now that I checked every component and replaced every E cap and Transistor. So I decided to go for it. I put the outputs in (Brand new). Turned bias down, no load, no signal, and powered up. No fuse blowing this time No Weird noise.

I measured across J2. Got 0.000V. I started to creep up R21 super slow. Nothing, Nothing, Nothing.....I chickened out and shut it down. Last time I turned it up it blew. I know I changed that fuse but I still didn't feel safe doing it. Then I remembered the factory had used that black nail polish to mark/hold the pot. I decided it wasn't that much further away from where I chickened out at so I decided to try it right that. 0.001mv. Aha....so very very slowly I got it up to where it's just flickering the meter between 0.004 and 0.005mv which should be .005mv exactly (on my non calibrated meter).

Most references and Wawa say .008 -.020mv. Manufacturer says .005v. I'm going to go with the safer number. My plan is to let it run a while and monitor the heat sink. So far it seems room temp or slightly higher. Going to use my laser temp probe on it Then I will try plunging in the preamp and see what happens. Then I will try a small 1khz Signal and try to look for crossover distortion. Then I will try testing it at a higher power and let it go for a bit and make sure there are no overheating or other distortion issues. Will update then.

jarrod0987

Another Q regarding this amp. I am using the grey rubberized thermal covers on my output transistors instead of grease, couldn't get mica in that size and I hate it anyways. I just read an article that says silicone thermal pads are useless at high power. This is a 100w amp. Am I using the wrong thermal product?

What is a normal temp range for the heat sink?I have been running it for a while with no signal and pre amp. Ambient is about 67F(19C) and the warmer (driver) side of the heat sink is 86F(30.1C) but of course I am not making the amp do anything right now.

Also...When I scope it I have been using the ground side of the output for the ground clip on my oscope. I tried using earth ground but it was really noisy. However, all the articles say use earth ground. Am I doing that wrong too?

jarrod0987

I measured the x4 .1ohm resistors and I know they are right on spec. SO I ran the calculations and this is below 20ma which would be 8mv. I just want to be safe and not blow it up until I can get it on the scope.

I just got done testing it for temperature stability and idle and also have measured only 16mv between ground and output prongs on the output jack. It seems to stay 4mv bias until it warms up a bit. Once it stabalizes I get about 5mv.

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Bias is like the idle RPM of a car. Low, but ready to take off quickly: Class ABTurning the car off while waiting for a trafic light: Class B

I thought this was the other way around?

I remember watching a vid that showed class B as having both transistors on when the signal was close to Ov which was wasteful and inefficient. SO they added diodes (in theis case the BE diode's of the preceding stage) which fixed the problem? I wish I could find the link to that vid now but of course I can't.

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Bias current is measured WITHOUT input signal.Re-adjust the bias after 20 minutes of warm-up time.

Will do that right now. I was pretty sure it was without 1khz but what does it hurt to ask. It is confusing in the drawing because they put the bias procedure right next to a note about test conditions etc. I know Bias is DC preliminary set up before the signal comes. Just wanted to make sure.

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NEVER run the amp, or adjust bias, when the power transistors are not mounted on a heatsink !!!Thermal runaway kills the power stage very quickly.

No of course not. What I meant was that I didn't want to run it with the heat sink off while I had the 220 resistors in instead. Do you count drivers as Power transistors? I thought you meant the final output transistors.

So far I plugged in the pre amp and no explosions. Going to let it run for 20 then check the bias as you suggested, then time for the scope and a signal.

Thank You so much for all your help. I know a lot of these questions are amateurish but it has been a very long time and I am used to class A amps and logic gates. This AB audio stuff is more different then I thought.

Thank YouP.S. Is there any good books or DVD's on this subject. All I can find are a few design books. I would like repair books. Anything come to mind?

jarrod0987

So far it seems fixed, I scoped it, I can see no cross over distortion at all. All the knobs seem to do what they should. Running it at mediocre volume for a while and keep an eye on the heat sink temp.

Thank You for all your input. I learned a ton. I wish I knew where to start reading to get all that knowledge of yours. YouTube and Amazon don;t seem to go real deep in Guitar amp subject. My customer/Friend is going to be ecstatic. He bought this thing broken 7 years Ago (I think) just because of the name on it. Now he gets to have it work. Too bad It needs a new speaker too

This was a great post, l leaning a lot, thanks to you guys,.I have a question.... in this case the amp has only 1 transistor per side, for testing reason you replace the power transistor by a 200 ohms resistor right?.... Now if you have more than 1 transistor 2, 3, 4, etc per side, do i have to replace for testing reason all the transistors by 1,2,3, etc 200 ohms resistors, that means per every transistor 1 resistor?..

This thread is over two years old, its a big ask to expect people to read 50 odd postings from years agoto understand the context to your question... Either start a new thread explaining the hardware andsoftware afresh, or point us to which postings in this thread are relevant to your question.

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Electronic Technician, Electronic Engineering Technician
I love to build things. Test equipment, replica and original sci fi props and costume pieces, and whatever else I feel like at the time. I have an Ultimaker and a 3D engraver.